1. Field of the Invention
The present invention relates generally to a communication system using an Orthogonal Frequency Division Multiple Access (OFDMA) scheme (hereafter referred to as an “OFDMA communication system”), and in particular, to a method for allocating resources for data transmission and an apparatus for transmitting and receiving data using the allocated resources.
2. Description of the Related Art
An Orthogonal Frequency Division Multiplexing (OFDM) scheme can be defined as a two-dimensional access scheme obtained by combining time division access technology and frequency division access technology. An OFDM symbol generated by the OFDM scheme is carried by a plurality of subcarriers after being divided, and the subcarriers are grouped into a subchannel before being transmitted.
The OFDM scheme, in which subchannel spectra overlap each other while maintaining mutual orthogonality, has high spectrum efficiency, and because OFDM modulation and demodulation is performed by Inverse Fast Fourier Transform (IFFT) and Fast Fourier Transform (FFT), a modulation/demodulation block can be subject to efficient digital implementation. In addition, the OFDM scheme, which is robust against frequency selective fading or narrowband interference, is efficient for transmitting high-speed data, and has now been adopted as a standard for transmission of European digital broadcasting and for high-capacity wireless communication systems such as IEEE 802.11a, IEEE 802.16a, and IEEE 802.16b communication systems.
The OFDM scheme is a kind of Multi-Carrier Modulation (MCM) scheme. The MCM scheme converts a serial input symbol stream into parallel symbol streams and modulates the individual symbol streams with a plurality of orthogonal subcarriers (or subchannels) before transmission. The MCM scheme was first applied to military high-frequency radio communications, and has developed into a scheme for overlapping a plurality of orthogonal subcarriers.
However, the OFDM scheme had a limitation in being actually applied to a system because it should be able to implement orthogonal modulation between multiple subcarriers. Thereafter, the rapid progress of technical development of the OFDM scheme was triggered by the announcement of a Discrete Fourier Transform (DFT) technique that can efficiently perform the OFDM modulation and demodulation. In addition, the use of a guard interval and the insertion of a cyclic prefix (CP) into the guard interval have reduced the defects of the system caused by multipath propagation and delay spread.
Therefore, the OFDM scheme is now widely being applied to such digital transmission technologies as digital audio broadcasting (DAB), digital television (DTV), wireless local area network (W-LAN), and wireless asynchronous transfer mode (W-ATM). That is, the OFDM scheme, which was not widely used due to its hardware complexity, can now be implemented because of the recent developments in the various digital signal processing technologies including FFT and IFFT.
The OFDM scheme, being similar to the conventional frequency division multiplexing (FDM) scheme, is characterized by transmitting a plurality of subcarriers while maintaining orthogonality therebetween, thereby obtaining the optimal transmission efficiency during high-speed data transmission. In addition, the OFDM scheme has high frequency efficiency and is robust against multipath fading, so it can obtain the optimal transmission efficiency during high-speed data transmission. Because the OFDM scheme overlaps frequency spectrums, it has high frequency efficiency and is robust against frequency selective fading and multipath fading. In addition, the OFDM scheme can reduce inter-symbol interference (ISI) with the use of a guard interval. Moreover, the OFDM scheme enables a simple hardware design of an equalizer. Further, the OFDM scheme is robust against impulse noises. Therefore, the OFDM scheme is now widely being applied to the communication system.
An Orthogonal Frequency Division Multiple Access (OFDMA) scheme is a multiple access scheme based on the OFDM scheme. In the OFDMA scheme, a plurality of users, i.e., a plurality of terminals, individually use subcarriers in one OFDM symbol.
A mobile communication system using the foregoing conventional OFDM scheme fixes time and frequency channels allocated to users like OFDM-Time Division Multiple Access (TDMA) or OFDM-Frequency Division Multiple Access (FDMA) in a cellular mobile communication system. That is, the OFDM scheme means that users using OFDM and multiple access technologies in one cell perform TDMA or FDMA transmission. Therefore, in order to increase frequency efficiency, the same frequency bands are reused by a plurality of cells.
A degree of the frequency reuse is determined by a frequency reuse factor. Commonly, the frequency reuse factor is fixed to 3, 4 and 7, so frequency reuse efficiency is not so high (frequency reuse factor is greater than 1) due to the fixed channel allocation technologies, and allocation of fixed subchannels leads to a high bit error rate (BER) due to frequency selective fading channels.
Therefore, broadband wireless access (BWA) technologies based on a method of balancing interferences (interference balancing technology) have been proposed. Band Division Multiple Access (BDMA) technology and Multi-carrier Code Division Multiple Access (MCDMA) technology are typical examples of the interference balancing technology. Herein, interference balancing is achieved by a diversity effect of interference occurring due to intercell random frequency hopping and spread spectrum technologies.
The interference balancing technology can show better performance, compared with the fixed channel allocation technology. However, the interference balancing technology cannot perfectly accomplish advantages of multicarrier modulation such as multiuser diversity and adaptive resource allocation with channel information in base stations (BSs).
To address such problems, interference avoiding technology such as dynamic channel allocation has been proposed. The interference avoiding technology can be two or three times higher than the interference balancing technology in terms of frequency utility. Therefore, combination of OFDM and multiantenna technologies and dynamic subchannel allocation technology based on adaptive modulation and low-complexity interference avoiding technology noticeably reduces the influence of deep fading and co-channel interference (CCI), thereby increasing frequency efficiency and system capacity.
A frequency hopping (FH) scheme, which is a scheme for dynamically changing subcarriers allocated to a particular terminal, is a typical example of the foregoing interference avoiding technology. An FH-OFDMA scheme is obtained by combining the FH scheme and the OFDMA scheme. A communication system using the FH-OFDMA scheme (hereinafter referred to as an “FH-OFDMA communication system”) enables a frequency band of subcarriers allocated to terminals to hop at predetermined intervals. That is, the FH-OFDMA communication system also attempts to acquire frequency diversity gain by distributing all of the subcarriers, i.e., data subcarriers over the full frequency band.
In a cellular system based on the FH-OFDMA scheme, every cell or sector balances interference from a neighboring cell or sector using an independent subcarrier allocation pattern. Such a technique is different in interference characteristics from the interference balancing technology for a CDMA scheme in which transmission signals exist in the same frequency band and spreading/despreading is performed through codes because the FH-OFDMA system uses only a part of a frequency band, unlike the CDMA system that uses all of a particular frequency band. Therefore, if a particular subcarrier is used even in a neighboring cell, the FH-OFDMA system suffers severe performance degradation in an uplink, compared with the CDMA system.
In addition, the CDMA system allocates the same frequency band to each sector. In this case, the influence of interference can be reduced by balancing the interference by spreading/despreading of codes. If this is applied to the OFDMA system, performance degradation occurs in a location where neighboring sectors or neighboring cells use the same subcarrier. The performance degradation is severe, especially for the uplink. In order to solve this problem, there is a need for a cell structure having a new concept being different from the sectorization concept used in the conventional CDMA system.
In order to propose the new cell structure, an efficient resource allocation scheme for each cell or sectors constituting each cell should be taken into consideration along with mobility of terminals. Herein, the efficient resource allocation means high frequency efficiency, i.e., a frequency reuse factor approximating 1. It is common that closer the frequency reuse factor is to 1, the higher the frequency efficiency is.